1. Field of the Invention
The present invention relates to methods for processing semiconductor substrates and the semiconductor substrates, and particularly to a semiconductor substrate processing method and a semiconductor substrate in which formation of dust particles from the edge part of the substrate is prevented.
2. Description of the Background Art
While SOI (Silicon On Insulator) devices in which semiconductor elements are formed on SOI substrates are superior to bulk devices in their reduced junction capacitance and improved element isolation breakdown voltage, the SOI devices have their unique problems as described below.
FIG. 32 shows a partial cross-section of an SOI substrate 10. The SOI substrate 10 includes a buried oxide film 2 and a single-crystal silicon layer (hereinafter referred to as an SOI layer) 1 sequentially stacked in the upper main surface of a single-crystal silicon substrate 3.
SOI substrate manufacturing methods include an SIMOX (Separation by Implanted Oxygen) method and a bonding method, for example. The SOI substrate 10 shown in FIG. 32 is manufactured by the SIMOX method.
In the SIMOX method, an SOI structure is obtained by implanting oxygen ions into a single-crystal silicon substrate to doses of 1.times.10.sup.18 to 2.times.10.sup.18 /cm.sup.2 at 150 to 200 KeV and then annealing it at about 1300 to 1400.degree. C., for example.
FIG. 32 shows the edge part of the SOI substrate 10 in detail. In the following description, a semiconductor substrate is referred to separately in its upper main surface (the side on which semiconductor elements are formed), center part thereof (the part including the active region), edge part including the peripheral part around the center part and the side part, and lower main surface.
As shown in FIG. 32, the edge part is curved with a large curvature. Accordingly, when oxygen ions are implanted there from the vertical direction, the oxygen ions are obliquely implanted, so that the effective implantation energy is reduced. As a result, the buried oxide film 2 and the SOI layer 1 are thinner in the edge part. Further, the surface of the edge part is not smooth but rough with irregularities. This phenomenon is general with silicon substrates formed by CZ (Czochralski) method. In the irregular part, the SOI layer 1 may be so thin that the buried oxide film 2 is exposed. In this condition, the SOI layer 1 is prone to exfoliation.
In addition, the film-thinning process for the SOI layer 1 performed in the SOI device manufacturing process facilitates the exfoliation of the SOI layer 1. The film-thinning process for the SOI layer 1 will now be described.
The SOI layer 1 in the SOI substrate 10 is formed to an approximate thickness at the time of production of the substrate. The film-thinning process for the SOI layer 1 is performed to appropriately reduce the thickness of the SOI layer 1 according to specifications of desired semiconductor devices. In this process, the thickness of the SOI layer 1 is adjusted by oxidizing the SOI layer 1.
The thickness of an oxide film formed on the SOI layer is generally determined on the basis of the thickness of the SOI layer 1 in the center part of the SOI substrate 10, or in the semiconductor element formation region (active region). In this process, the thin SOI layer 1 in the edge part of the SOI substratelo presents the problem as stated above. The buried oxide film 2 may be exposed in some parts.
FIG. 33 is a schematic diagram showing the region X in FIG. 32. As shown in FIG. 33, the buried oxide film 2 is also irregular in the edge part of the SOI substrate 10, reflecting the shape of the irregular part DP of the SOI layer 1. Since oxygen ions are implanted from the vertical direction, the irregularities on the SOI layer 1 and the irregularities on the buried oxide film 2 are formed in shifted positions, which may cause the buried oxide film 2 to be exposed.
Next, FIG. 34 shows the SOI layer 1 and an oxide film OX formed thereon to thin the SOI layer 1. As the formation of the oxide film OX thins the SOI layer 1, the oxide film OX may be coupled to the buried oxide film 2 or the SOI layer 1 may be completely oxidized in the edge part. In such a case, the SOI layer 1 may be partially surrounded by the buried oxide film 2 and the oxide film OX. For example, the part 1A of the SOI layer shown in FIG. 34 is surrounded by the oxide film OX and the buried oxide film 2.
When the SOI substrate 10 is wet-etched in this condition with an etching solution, such as hydrofluoric acid (HF), to remove the oxide film OX, the buried oxide film 2 will be etched away together with the oxide film OX, as shown in FIG. 35. Then, the SOI layer 1A will be lifted off to be a particle, which will be suspended in the etching solution and may adhere to the center part of the SOI substrate 10. If particles adhere to the semiconductor element formation region, it will cause defective formation of semiconductor elements to reduce the production yield.
In some cases, a polysilicon layer may be formed on the edge part and the lower main surface of the silicon substrate 3 for gettering of contaminants like heavy metals taken in the wafer manufacturing process or in the wafer process for transistors. In this case, the SOI layer 1 and the buried oxide layer 2 become uneven due to polycrystallinity of the polysilicon layer, and the SOI layer 1 will then partially come off to be particles, similarly to the phenomenon described above.
Particles may be produced also with SOI substrates formed by a bonding method (bonded substrates).
With a bonded substrate, the SOI structure is obtained by forming an oxide film on the upper main surface (on the main surface on which semiconductor elements are formed) of a silicon substrate 3, bonding another silicon substrate thereon, and polishing that silicon substrate to a desired thickness. FIG. 36 shows a cross-section of the edge part of an SOI substrate 20 formed this way.
In FIG. 36, an on-substrate oxide film 6 and a silicon layer 7 are placed on top of the other on the upper main surface of the silicon substrate 3 to form an SOI structure. The on-substrate oxide film 6 corresponds to the buried oxide film and the silicon layer 7 to the SOI layer.
With the SOI substrate 20 having this structure, the on-substrate oxide film 6 is exposed in the edge part. Hence, etching solution used in wet-etching may invade the exposed part to partially remove the on-substrate oxide film 6, causing the silicon layer 7 to be partially hung as shown in FIG. 37. In this condition, the silicon layer 7 is prone to come off to be a particle.
When the edge part of the on-substrate oxide film 6 and the silicon layer 7 is imperfectly beveled, the periphery will show continuous irregularities in a plane view. The irregular part may come off in transportation of the substrate to produce particles.
As described above, conventional semiconductor substrates, particularly SOI substrates produced by the SIMOX method have the problem that the SOI layers in the edge part of the substrates may come off to be particles, to reduce the production yield. Particles may be produced also with SOI substrates manufactured by the bonding method.